We studied the mechanism of low back pain using rats, and obtained the following results.(1) Fos expression in the brain and spinal cord in condition of low back pain : Acute noxious stimulation delivered to lumbar muscles (deep pain group) and skin (cutaneous pain group) of rats was used to study Fos expression patterns in the brain and spinal cord. In the spinal cord, Fos-immunoreactive (Fos-ir) neurons were not observed in the lamina II after stimulation of low back muscles. In the brain, Fors-ir neurons were observed in significantly greater numbers in the deep somatic pain group in the accumbens, basolateral nucleus of amygdala, paraventricular hypothalamus, and the ventrolateral periaqueductal gray than in the cutaneous pain group.(2) Substance P and Calcitonin Gene-Related Peptide immunoreactive sensory dorsal root ganglion (DRG)neurons innervating the lumbar facet joints in rats : Neurons innervating the L5/6 facet joint were distributed throughout DRGs from L1 to L5 levels. The ratios of SP and CGRP-ir neurons inL1 and L2 DRGs were significantly less than those in L3, L4 or L5 DRGs. In the physiological condition in rats, lower DRG neurons may have a more significant role in pain sensation of the facests than upper DRG neurons.(3) DRG neurons with dichotomizing axons to the facet joint and sciatic nerve : We demonstrated in rats the existence of the sensory neuron with dichotomizing axons both to the facet joint and the sciatic nerve using double-labelling method. This neuron provides a possible neuro-anatomical substratum for referred leg pain in facet joint disease.